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1
Place the pork belly blocks in a wide saucepan and cover with cold water. Bring to a full boil and cook for 10 minutes. Drain, discard the blanching water, and rinse the blocks under cold running water. The surface of each block should be pale and clean, free of grey foam or residue.
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2
Return the rinsed pork blocks to the pan in a single layer. Add the dashi and sake. Bring to a gentle simmer over medium heat, then reduce the heat until the surface barely moves — a visible but not rolling simmer, approximately 90–92°C. Set an otoshibuta (drop lid) directly on the surface of the liquid to maintain even contact. Hold at this temperature for 2 hours 30 minutes. The blocks should hold their squared shape throughout; a vigorous boil is the point at which the dish is lost.
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3
After 2 hours 30 minutes, add the mirin, soy sauce, and sugar to the braising liquid. Continue the gentle simmer for a further 30 minutes. The liquid will deepen in colour and the surface of each block will take on a lacquered sheen. Probe a block with a chopstick or skewer: it should pass through with no resistance, and the block should quiver when the pan is moved.
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4
Remove the pan from the heat and allow the kakuni to cool in the braising liquid for at least 30 minutes. As the liquid cools, the blocks will absorb the broth, intensifying their flavour. The liquid around the blocks will thicken perceptibly as the dissolved gelatin sets at the surface.
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5
Lift the kakuni blocks from the braising liquid and set aside. Pour the remaining liquid into a small saucepan and reduce over medium heat until it reaches a syrupy, glaze consistency — it should coat the back of a spoon heavily and hold a line when a finger is drawn across it.
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6
Warm the kakuni blocks gently in the reduced glaze. Plate each block and spoon the glaze directly over. Serve with a portion of karashi mustard alongside. The finished block should hold its square shape on the plate while yielding completely to the gentlest pressure.